1. Field of the Invention
The invention relates to a process for producing molten pig iron or molten steel pre-products from charging substances formed of iron ores and fluxes and at least partially comprising fines, wherein the charging substances are directly reduced to sponge iron in at least one reduction zone by the fluidized layer method, the sponge iron is melted in a melting-gasifying zone under supply of carbon carriers and oxygen-containing gas, and a CO and H.sub.2 -containing reducing gas is produced, which is injected into the reduction zone, is reacted there, is withdrawn as an export gas and is supplied to a consumer, as well as a plant for carrying out the process.
2. Description of the Related Prior Art
A process of this kind is known, for instance, from Austrian Patent AT-B 390 622. According to AT-B 390 622, charging substances having largely varying grain sizes are processed, the charging substances being pre-reduced and separated by wind screening into fractions of different grain sizes, which are then completely reduced separately. However, this known one-step process only offers a low thermal utilization of the reducing gas and consequently involves an elevated consumption of reducing gas. Nor is the optimum utilization of the energy chemically bound in the reducing gas feasible.
According to Austrian Patent AT-B 387 403, siderite-containing and/or hydrated charging substances are calcined in a fixed-bed heating zone preceding the fixed-bed direct reduction zone, wherein, however, only coarse lumps of iron-ore-containing charging substances capable of being processed merely in the fixed bed are used for charging.
From U.S. Pat. No. A 5,082,251, a direct reduction process is known, according to which fine ores rich in iron are reduced after complex ore preparation, such as drying, screening and breaking, in a system of whirl layer reactors in cascade arrangements by aid of reformed natural gas or oil so as to obtain a very narrow grain size distribution. Subsequently, the iron powder is hot- or cold-briquetted. Smoke gas is used as the fluidizing gas in the preheating stage, which is produced by burning air and natural gas; thus, external energy must be introduced, only the sensible heat of the whirling gases being utilizable. In contrast, reduction according to the process of the present invention is effected by means of solid carbon carriers, such as coal, and hence, according to the invention, CO reduction is preponderant, whereas, according to U.S. Pat. No. A 5,082,251, the direct reduction of ore primarily is effected by H.sub.2.
The invention has as its object to provide a process of the initially defined kind as well as a plant for carrying out the process, which enable the use of iron ores and fluxes comprising at least a share of fines, in an economic manner by using untreated coal as a carbon carrier, wherein the chemically bound energy (CO, H.sub.2 -content) still contained in the reducing gas used can be utilized.
In accordance with the invention, this object is achieved with a process of the initially defined kind in that
primarily hematite and/or magnetite fine ores and/or ore dusts are subjected to preheating by the fluidized layer method in a preheating zone, PA1 the thus preheated charging substances are completely reduced to a major extent in at least one consecutively arranged reduction zone, PA1 whereupon at least the more finely particulate charging substances are charged into the fluidized bed and/or, if desired, also into the fixed bed, of the melting-gasifying zone by forced conveyance, preferably by pneumatic conveyance, and are melted there.
It is essential to the present invention that the charging substances are processed not in the material counterflow as is the case with the known fixed-bed methods (AT-B 387 403), but in stable or circulating fluidized layers, i.e., for instance, in diagonal flow, thus enabling the economic processing of fine ores and ore dusts on account of the improved energetic gas utilization. This is of importance, because, at present, about 75% of the world's ores incurs as fine ore, which is cheaper than lumpy ore or agglomerates. According to the invention, not only reduction is effected by the fluidized layer method, but also preheating. By the multi-step fluidized layer method according to the invention it has become possible to use the reducing gas in an optimum manner without having to feed additional energy.
A substantial advantage of the process according to the invention is to be seen in that ore preheating is effected by means of process reducing gas from the final reduction stage and not by external gas supply as according to U.S. Pat. No. A-5,082,251, which, of course, involves accordingly high costs. Another advantage of the gas control implied by the present invention resides in that pre-reduction can be achieved by the reducing atmosphere in addition to preheating, a particularly efficient utilization of the reducing gases, thus, being ensured.
To cool the reducing gas formed in the melting-gasifying zone, the reducing gas, according to the invention, partially is fed directly to the reduction zone for forming a fluidized layer and partially, after purification in a hot cyclone and in a scrubber, is admixed as a cooling gas to the first portion of the reducing gas fed to the reduction zone.
To control the state of fluidization of the charging substances in the reducing zone, a portion of the reducing gas advantageously is fed to the reduction zone in the region of the fluidized layer and part of the portion of the reducing gas supplied to the hot cyclone is fed to the reduction zone into a fluidized bed formed in the lower part thereof.
To efficiently preheat the charging substances, the reducing gas leaving the reduction zone advantageously is fed to the preheating zone, a temperature increase being effected by the partial combustion of the reducing gas.
To efficiently use the dust and fines incurring in reduction, the reducing gas withdrawn from the reduction zone advantagously is freed from fines in a reduction cyclone and the fines separated in the reduction cyclone are completely reduced to a major extent during separation and are supplied by means of an injector to the melting-gasifying zone in the region of feeding of oxygen-containing gas.
Fines that have been completely reduced in the reduction zone already prematurely, suitably are partially discharged from the fluidized layer of the reduction zone and are supplied by means of an injector to the melting-gasifying zone in the region of feeding of oxygen-containing gas via a sleuce system, the portion of charging substances discharged from the fluidized layer of the reduction zone suitably being supplied to the melting-gasifying zone together with the material separated in the reduction cyclone.
In doing so, the dust separated in the hot cyclone advantageously is supplied to the melting-gasifying zone in the region between a fine-coke fluidized bed forming there and a coarse-coke fluidized bed, via a sleuce system by aid of an injector and by means of an oxygen dust burner.
Suitably, the addition of fluxes is effected by charging a portion of the fluxes required for the melting process, together with the coal, directly into the melting-gasifying zone and a portion of the fluxes, together with the fine ore, into the preheating zone, wherein, advantageously, the fluxes charged together with the coal are introduced as coarse grains, preferably ranging between 4 mm and 12.7 mm, and the fluxes charged together with the fine ore are introduced in a fine-grain form, preferably ranging between 2 mm and 6.3 mm.
Particularly efficient reduction may be obtained by providing two locally separated consecutively arranged reduction zones, the reducing gas leaving the first reduction zone being conducted to the second reduction zone preceding the first reduction zone in the sense of the fine ore flow and from there being fed to the preheating zone under compression.
To utilize the excess gas incurring in the process, the export gas leaving the preheating zone, according to a preferred embodiment, if desired, upon admixture of a portion of the reducing gas leaving the reduction zone, after CO.sub.2 purification, is used for producing hot-briquetted iron, wherein fine ore is subjected to preheating in a preheating zone, subsequently is subjected to a largely complete reduction in at least one reduction zone and, furthermore, is supplied to a compressing and briquetting means, and the export gas, upon heating, is conducted into the at least one reduction zone under formation of a fluidized bed, and, after having flown therethrough, is withdrawn from the same and is fed to the preheating zone under partial combustion with a view to temperature elevation for the purpose of forming a fluidized bed.
An arrangement for carrying out the process according to the invention, comprising at least one reduction reactor, into which a conveying duct for charging substances containing iron ore and fluxes, a gas duct for a reducing gas as well as a conveying duct for the reduction product formed therein and a gas duct for the export gas enter, and comprising a melter gasifier, into which the conveying duct conducting the reduction product from the reduction reactor enters and which comprises feed ducts for oxygen-containing gases and carbon carriers as well as taps for pig iron or steel pre-material and slag, wherein the gas duct for reducing gas formed in the melter gasifier entering the reduction reactor departs from the melter gasifier, is characterized in that the reduction reactor is designed as a fluidized-layer reduction reactor and that, in the flow direction of the charging substances, a fluidized-layer preheating reactor precedes the fluidized-layer reduction reactor, the gas duct of the fluidized-layer reduction reactor entering into the fluidized-layer preheating reactor, and that a pneumatic conveying duct is provided for conveying the sponge iron formed in the fluidized-layer reduction reactor into the melter gasifier, the conveying duct entering the melter gasifier on the level of the fluidized bed and/or fixed bed.
The reduction process may be controlled via the degree of fluidization prevailing within the reduction reactor (and also within the preheating reactor) advantageously in that the fluidized-layer reduction reactor comprises a lower part having a smaller diameter and an upper part following upon the lower part and having a larger diameter, the transition from the lower part to the upper part being conically designed and the gas duct for the reducing gas entering the conical transition piece, wherein the fluidized-layer preheating reactor suitably has a conical lower end into which the gas duct for the reducing gas runs.
In order to be able to discharge completely reduced fines from the fluidized-layer reduction reactor, the fluidized-layer reduction reactor, on the level of the fluidized layer, is provided with a fines discharge means, from which a conveying means leads to a pneumatic conveying means entering into the melter gasifier on the level of the fixed bed or fluidized bed formed therein.
According to a preferred embodiment, two fluidized-layer reduction reactors are consecutively provided in the flow direction of the charging substances.
A particularly efficient utilization of the excess gases forming is provided if the gas duct for the export gas, after the intermediate arrangement of a CO.sub.2 scrubber and a heating means, runs into at least one reduction reactor for producing hot-briquetted iron, from which reduction reactor a gas duct is conducted into a fluidized-layer preheating reactor, wherein a fine-ore charging duct enters into the fluidized-layer preheating reactor and a conveying duct departs from the fluidized-layer preheating reactor, conducting the preheated fine ore to the reduction reactor, and if a compressing and briquetting means is arranged to follow the reduction reactor in the direction of the fine-ore flow.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.